While in principle EPR imaging can provide a comprehensive description of the distribution of pO2, this approach usually is limited by the sensitivity of EPR measurements. Localized EPR spectroscopy with particulate materials can provide long term measurements of pO2 in tissue at several sites simultaneously. Magnetic field gradients along X-, Y- and Z-axises were used to resolve EPR spectra from several different sites simultaneously. Using rat hearts and tumors with placement of pieces of material at a distance six or more times the size of the pieces themselves (no matter what kind of material is used), multisite measurements of pO2 (up to eight sites) have been carried out. For example, with LiPc crystals of about 0.5mm in size, sites separated by 3mm or more could be readily followed.